The invention is concerned with a water treatment device of the kind used in treating water in for example a swimming pool or other body of water (referred to herein as a "swimming pool" for convenience), the device comprising a water treatment pill i.e. a pill which has a cylindrical periphery and disk-like end faces and which comprises material that in use is slowly eroded to treat the water. Such a device is hereinafter referred to as "a pill".
Particularly desirable forms of pill comprise solid trichloroisocyanuric acid (hereinafter referred to as "TCIA") or calcium hypochlorite ("CHC") as the erodable material.
It is important with pills that the erosion of the pill should be constant or substantially constant so that there is a constant rate of water treatment. Attempts have been made to control the rate of dissolution of the tablet including that shown in U.S. Pat. No. 4,546,503. In that specification, a pill is shown having a frusto-conical lower portion and an enlarged bore through the center. The pill is housed in a container 11. There is water contact between the water and virtually the entire exposed surface of the pill including the surfaces surrounding the bore. Because of this large area of contact between the water and the pill, erosion is accelerated. Further, erosion of the pill takes place in a non-uniform manner. Another way of controlling erosion is for only the disk like ends of the pill to be exposed to the water because there is then a constant area of the pill in contact with the water irrespective of the thickness (or axial height) of the pill. Consequently the cylindrical periphery of the pill should be covered to prevent water coming into contact with the periphery of the pill. British patent specification 1 229 081 (May) shows such an arrangement. Other similar arrangements are shown in U.S. Pat. Nos. 4,876,003 and 4,928,813 (both Casberg) although in the latter cases the whole end of the pills are not exposed, initially at least, so that erosion takes place in a fairly uneven manner as illustrated in FIG. 4 of U.S. Pat. No. 4,876,003. Furthermore the arrangement disclosed in both these specifications very large and specially made pills are required which complicates and increases the cost of manufacture. These pills can in practice only be used in a weir (or skimmer) where the high flow rate will erode CHC far too quickly for the pills to be practical.
It is often desirable to mount the pills with their axes vertical. Reasons for this include the desirability to make the container for the pills as shallow as possible particularly when a plurality of pills are to be mounted in the container. Furthermore with the pills so mounted, the erosion pattern is more constant than when the pills are mounted with their axes horizontal. In such circumstances, a problem arises in that as the pill erodes, a bubble of gas is often formed below the lower disk-like surface of the pill. This has two major disadvantages. First, the undersurface of the pill will not be in contact with the water and therefore will not be eroded thereby. This is disadvantageous where the pill has both surfaces nominally exposed to the water but is more serious if only the under surface is exposed as mentioned below. Furthermore the location of a bubble of gas underneath the pill will have a buoyancy effect which will tend to raise a housing containing the pill. This could cause the housing to tip or to invert which may in certain circumstances have unwanted and possibly harmful consequences.